====== L2Dist ======

compute the L2 Distance between two fields

options
<code>
  -sx       --shiftx                   shift by Lx/2
  -sz       --shiftz                   shift by Lz/2
  <flowfield>      (trailing arg 2)    input field #1
  <flowfield>      (trailing arg 1)    input field #2
</code>

usage
<code>
L2Dist u.ff v.ff
</code>

returns 

<latex>
 \text{L2Norm(u-v)} = \|{\bf u} - {\bf v}\|  = \left(\frac{1}{L_x L_y L_z} \int_{\Omega} \|{\bf u} - {\bf v}|^2\,\, d{\bf x} \right)^{1/2}
</latex>

Note: the -sx and -sz options apply shifts by Lx/2 and Lz/2 to one of the fields prior to computation of the norm.
This is helpful for my research but might not be of interest to others.